An apparatus is disclosed in EP-A0 269 561 wherein a pipetting insert is inserted into the stopper of the sample vessel by means of a plunger. The disadvantage of such apparatus is that in some cases, depending on the nature of the material of the stopper, the constant pressure forced exerted by the plunger on the pipetting insert on introduction of the latter into the stopper may not be sufficient to insert the pipetting insert completely or at least sufficiently deeply into the stopper. The elastic deformation of the stopper during the introduction of the pipetting insert into it reverses to varying degrees after elimination of the pressure applied to the pipetting insert, and tends to displace the pipetting insert from the stopper. The resulting incomplete introduction of the pipetting insert into the stopper jeopardizes the reliability of the apparatus for introduction of the pipetting inserts.